Bacillus velezensis supernatant mitigates tomato Fusarium wilt and affects the functional microbial structure in the rhizosphere in a concentration-dependent manner

This study aimed to test the hypothesis whether the soil-application of Bacillus velezensis Bs006 and its supernatant affect the rhizospheric microbial community of tomato-Fusarium oxysporum f. sp. lycopersici pathosystem. The relationship between the functional response of the microorganisms measured through use of carbon sources and the disease development was determined under a dose-response test between Fusarium wilt development and Bs006-based treatments (spores at 1 × 10⁷ and 1 × 10⁹ cfu ml⁻¹ and supernatant at 1 and 10%). Use of carbon sources by the microbial community in the rhizosphere at 3 and 30 days after treatment were determined through the Ecoplate Biolog® microplate assays. The efficacy against disease development was dependent on the concentrations of biocontrol treatments, with low concentrations (Supernatant-1% and Bs006-10⁷ cfu ml⁻¹) significantly reducing the incidence of tomato vascular wilt compared to the effect of the highest concentrations (Supernatant-10% and Bs006-10⁹ cfu ml⁻¹). The use of carbon sources expressed as the average metabolic response (AMR), by the rhizosphere microbial community was higher in the less effective treatments of biocontrol than the absolute control and the plants treated with low concentration of biocontrol treatments. The high AMR was related with low functional microbial diversity, expressed as the number of carbon sources consumed, which was also low in the rhizosphere of the negative control. On the contrary, the AMR was lower, and the functional microbial diversity was higher in the most effective treatments against Fusarium wilt. Functional microbial groups consuming carbohydrates, polymers and amines were associated with the less effective treatments, while the use of amino acids and carboxylic acids was associated with the most effective treatments. We conclude that treatments containing Supernatant-10% changed the rhizosphere microbial behavior for carbon sources consumption and it was stablished as a factor that promotes the tomato Fusarium-wilt development. This study contributes to the aspects to be considered in the generation of use directions of fungicides based on Bacillus spp.